Why Bloom Iron ?- five

(Day 17)

The second distinctive characteristic of bloomery iron is variation in carbon content.

(This is going to be a bit technical for the non-metalsmiths.)

Carbon has a major effect on iron when it is added (alloyed) to the metal. Even small amounts of carbon drastically effect the relative resistance to deforming (effectively the hardness). Some modern metals with their approximate carbon contents:
'Electric Iron' ( a low carbon Bessemer steel, used for transformer cores) - about 0.05 % Carbon
Mild steel (what most everything is made of, from cars to I beams) - about 0.2 % Carbon
Spring steel (old leaf and coil springs in autos, heavy cutting tools) - about 0.5 % Carbon
Tool steel (small, sharp cutting edges) - about 1.0 % Carbon
Cast Iron (cookware, stoves) - about 2.0 % Carbon

Because iron alloyed with carbon resists forming - it will get increasingly more difficult to actually forge (hot hammer) shapes. In the pre-Industrial Age world, the ideal iron for the blacksmith had as *little* carbon in it as possible.
You can see that this works in direct opposition to the requirements of the blade maker. For a durable cutting edge, you would want some amount of carbon present. Hardness equals edge holding ability. Also the complex series of heat treating methods apply to Carbon alloys. (A topic for *much* later!)


Remember our idealized direct process bloomery furnace:

Carbon is *not* present in the starting iron oxide ore.
Carbon may be absorbed by the reduced metallic iron in two ways:
First, directly to the surface of the individual particles as they fall down the body of the furnace. This is most commonly found with very small particle size ore, or with a furnace that is allowed to run too hot.

The second place carbon can come from is into the surface of the bloom, as it sits inside the liquid slag in the bowl at the bottom of the furnace. This is a slower process, so the carbon tends to diffuse from the outer layers towards the centre.
With larger blooms, the effective ratio of surface area to internal volume is lower. So this carbon variation effect tends to be more obvious with smaller blooms.
Also, the quality of the individual bloom comes into play. The folding and re-welding process will tend to 'average out' variations in carbon content ('carbon migration'). There can also be carbon absorbed from the fuel during repeated welding heats.

Ok - enough theory - what is the practical effect:

Bloom Buckle - Winter 2011
Forged from a small bloom fragment

The variation in colours seen over the surface of 'Bloom Buckle' is a direct result of variations in carbon content within the metal. As a small fragment, the original metal was more drastically effected by carbon diffusion across its surface (high area to volume ratio).
The finished buckle was water hardened, then etched with a Ferric Chloride solution to highlight the differences in carbon content. (This is a variation on the process used for my layered steel knives.)

This is another effect which I intend on exploring (if time permits) within the framework of the OAC Grant.


Note to Readers :
The Arts is well known for its (often inpennetrable ) jargon. You may have noticed I don't tent to use this 'Language of the Artist'. What you will find here is a tendency to technical jargon - itself often poorly understood (and frequently *improperly* used). I would refer you to my commentary piece 'Defining the Artist Blacksmith'.